Described herein are methods for forming multiple-layer electrode structures for silicon photovoltaic cells, that is, for silicon solar cells. The methods provide silicon solar cells having low contact resistance with a small contact area, and thus an improved contact resistivity over electrode structures formed with the use of glass frit. The methods also are able to provide reduced environmental concerns compared to the use of lead-based glass frit, a significant advancement as solar energy is a so-called “clean” energy.
Solar cells are typically photovoltaic devices that convert sunlight directly into electricity. Solar cells typically include a silicon semiconductor that absorbs light irradiation, such as sunlight, in a way that creates free electrons, which in turn are caused to flow in the presence of a built-in field to create direct current (hereinafter “DC”) power. The DC power generated by several photovoltaic (PV) cells may be collected on a grid associated with the cells. Current from multiple PV cells is then combined by series and parallel combinations into higher currents and voltages. The DC power thus collected may then be sent over wires, often many dozens or even hundreds of wires. The DC power may also be converted to AC power using well-known inverters.
To form metal contacts able to gather and convey the generated power, the solar cell material is metallized. For silicon solar cells, metallization typically comprises forming a grid-like metal contact, for example including fine fingers and larger busbars, on a front surface of the cell and forming a full area metal contact on the back surface of the cell. Conventionally, metallizing silicon solar cells is done by screen printing. Screen printing has been used for decades, and is a robust, simple, rapid, and cost-effective metallization method and can be easily automated for large-scale solar cell manufacturing. In a conventional screen printing approach to metallization solar cells, a squeegee presses a paste through a mesh with a pattern that is held over the wafer. A typical paste for solar cell metallization consists of a mixture of silver particles, a lead-based glass frit and an organic vehicle. When the wafer is fired (annealed), the organic vehicle decomposes and the lead-based glass frit softens and then dissolves the surface passivation layer, creating a pathway for the silver to reach the silicon base by forming a multitude of random points under a silver pattern formed by the paste. The surface passivation layer, which may also serve as an anti-reflection coating, is a dielectric layer, such as a silicon nitride layer, and is an essential part of the cell covering the cell except for electrical contact areas. Upper portions of the silver paste densify into one or more films that carries current from the cell. These films form gridlines on the front-side of the wafer, and a base contact on a backside of the wafer. The silver of the paste is also a surface to which tabs connecting adjacent cells may be soldered.
While the use of a lead-based glass frit combined with screen printing has advantages, it also has several drawbacks. First, contact resistivity, or specific contact resistance, is very large, for example, specific contact resistance between the semiconductor emitter layer (sun-exposed surface) and the silver gridline is on the order of about 10−3 Ω·cm2. This specific contact resistance between the silicon semiconductor emitter layer and the silver gridline is several orders of magnitude higher than the specific contact resistance that may be reached in semiconductor integrated circuit devices, which is on the order of about 10−7 Ω·cm2. The large contact resistivity is the result of a low effective contact area between the silicon semiconductor emitter layer and the silver electrode gridline due to the non-conductive glass frit occupying a considerable portion of the interface. Due to the large specific contact resistance, the emitter layer in a solar cell must be heavily doped and large contact area between the emitter and silver gridline must be used, otherwise the silver of the paste cannot make good electrical contact to the silicon. The heavy doping kills the minority carrier lifetime in the top portion of the cell and limits the blue response of the cell, and the large contact area generates higher surface recombination rate. As a result, the overall efficiency of the solar cell is reduced.
Another problem with the glass frit approach is a narrow process window. The narrow process window may be a problem because a thermal cycle, that fires the gridline, must burn through the silicon nitride to provide electrical contact between the silicon and the silver without allowing the silver to shunt or otherwise damage the junction. This narrow process window severely limits the process time to the order of about 30 seconds and temperature band to about 10° C. around the peak firing temperature.
Still further, the use of lead-based glass frit, required to dissolve portions of the passivation layer, raises environmental concerns.
Ideally, a metallization technology for silicon solar cells should form the gridline electrodes with low specific contact resistance and thus low contact area, high conductivity, good solderability, and long time stability. Because it is very difficult for a single layer electrode to meet all these requirements, several methods for forming multiple-layer electrode structures have been proposed for silicon solar cells.
U.S. Patent Application Publication No. 2007/0169806, incorporated herein by reference in its entirety, discloses forming multiple-layer gridline front surface electrodes by forming contact openings through the passivation layer using a non-contact patterning apparatus such as a laser-based patterning system. The contact openings may be filled by inkjet printed nanophase metallic inks and covered with silver gridlines. However, several problems associated with using printed nanophase metallic inks for filling the contact openings include quality and availability of the nanophase metallic inks, the wetting behavior and contact characteristic between the nanophase metallic ink and the silicon surface in the contact openings, and the process compatibility of nanophase metallic ink with firing silver gridlines.
U.S. Patent Application Publication No. 2004/0200520 discloses a multiple-layer backside electrode structure that is formed by making contact openings through chemically etching the passivation or anti-reflection coating layer, followed by sputtering or evaporating a three layer-seed metal stack to form the contact with emitter and plating copper and a thin metal capping layer to form gridlines. However, chemically etching the passivation layer involves several extra process steps including applying an etch resist layer, patterning the etching resist layer, and striping off the etching resist layer after patterning the passivation layer.
U.S. Patent Application Publication No. 2005/0022862 discloses screen printing a liquid ink pattern layer devoid of particles onto the silicon oxide passivation layer to form a particle-devoid ink pattern layer as an etching protection mask.
It is still deemed desirable to develop more cost efficient and/or less complex methods of forming metallization contact structures for solar cells that provide low contact resistance, low contact area, high conductivity, high solderability, and high stability from solar exposure.